1. Field of the Invention
This invention is concerned with a hot-cathode material in wire and sheet form and with a process for its production.
2. Description of the Prior Art
Hot-cathode materials for thermionic tubes are known in the art in numerous embodiments and with numerous combinations of materials. They range from the conventional oxide cathodes having a low operating temperature and a low emission current density but a long life, to complicated multi-material systems such as the so-called "reactive cathodes". The most widely used materials of the latter category are cathode materials of the type W/W.sub.2 C/ThO.sub.2 ("thoriated tungsten cathodes"), which operate on the basis of a chemical reaction and supply of the activator from the interior and which have a relatively high operating temperature. They are distinguished by a long life coupled with moderate emission current density. It can be demonstrated that the work function of the electrons can be lowered and that the electrode emission properties can be improved by adding a platinum metal to the above-mentioned system (see, for example, German Offenlegungsschrift No. 1,614,541). Materials are also known which have a medium life and a higher emission current density, examples of such materials being the systems Mo/Mo.sub.2 C/La.sub.2 O.sub.3 (see German Auslegeschrift No. 2,344,936) and Mo/Mo.sub.2 C/La.sub.2 O.sub.3 /Pt metal and the like (see German Auslegeschrift No. 2,454,569). Hot-cathode materials are also known which are based on porous sintered bodies produced from powder mixtures of a high melting metal with a platinum metal, the pores of the sintered bodies being filled with a material containing the activator (see, for example, German Offenlegungsschrift No. 2,727,187). Such cathodes are particularly distinguished by high emission current density coupled with relatively low operating tempertures.
The above-mentioned cathode materials are almost exclusively sintered materials which are used in the form of small sheets, tablets and similar compact components for the manufacture of hot-cathodes. These materials are distinguished by a certain brittleness so that, in general, they cannot be shaped or can only be shaped with extreme difficulty or by accepting substantial deterioration in their physical properties. Their lack of ductility does not enable them to be produced in any desired dimensions or, for example, to be converted economically to wires, sheets and strips, which would enable their high emission current density to be used in practice. On the other hand, the available materials with a high emission current density which can be produced in these physical forms have a service life that is still too short for practical use, particularly in the case of small sizes. There is, therefore, a requirement for materials which combine, in an optimum manner, the good properties of the above-mentioned materials and which allow the design engineer maximum possible freedom in design.